In an electromagnetic type fuel injection valve used in an internal combustion engine, in particular, a cylinder injection system for vehicle using gasoline, in order to satisfy the regulation or the demand with respect to exhaust gas or fuel consumption, there has been an increasing market demand for injecting fuel into an engine cylinder at a higher pressure compared to a conventional way of injecting. It is because the injection speed of fuel is increased and frictional, resistance between air and fuel becomes larger as the fuel pressure becomes higher, and therefore the fuel is further atomized, as a result, a combustion property is improved.
The electromagnetic type fuel injection valve is formed as a solenoid type valve which opens the valve, by attracting movable core formed as a movable iron core by a magnetic attraction force toward a fixed core formed as a fixed iron core to be collided with each other. In order to inject fuel at a high fuel pressure, large force to be applied to a valve body against the fuel pressure is required for opening the valve. Thus, impact load applied to a collision structural part of the fixed core and the movable core becomes larger compared to a conventional valve. In order to reduce wear and deformation of the collision structural part due to the load in colliding, a surface treatment with high strength is applied to the collision structural part.
Further, the collision structural part is a part of a magnetic circuit for generating the magnetic attraction force. It is preferable that facing surfaces of the fixed core and the movable core are arranged to be close in order to increase the magnetic attraction force. On the other hand, in order to improve a response property, it is preferable to enlarge each facing surface of the fixed core and the movable core. It is because the response property is improved by decreasing squeezing force as adhering force of liquid and increasing a closing valve speed. Thus, as described in JP 2011-89432 A, it is preferable to form a projection with small width and height on the facing surface of the fixed core or the movable core which are provided as the collision structural part, and therefore high processing accuracy is required because of its small shape.
Here, a thickness of the surface treatment might be uneven. When a partial projection is formed because of variation in thickness of the collision structural part, a stress concentration is occurred and therefore the wear is promoted.
In particular, in a case in which electroplating is used as the surface treatment, since magnetic flux density is uneven due to a shape of an object to be electroplated, the thickness of the surface treatment is apt to become uneven. Accordingly, in a conventional technique as described in JP 2011-89432 A, a taper is formed in advance on a base material to be electroplated.
When the wear and the deformation of the collision structural part become large, a stroke of the valve body becomes larger, a passage section becomes larger. This configuration leads to an increase of an injection amount and a decrease of the fuel consumption and the combustion property.